Background and Aim: One of the major factors affecting personnel is workload that puts many people at risk and may affect the quality and quantity of services. The object of this study was assessment of mental workload in teachers of Hashtrud city using NASA- TLX mental work load index. Materials and Methods: This was a descriptive study, which was conducted among 137 teachers of Hashtrud city in year 2013. In this study we used NASA-Task load indexes (TLX) for assessment of mental workload of teachers. Reliability and validity of this index were verified by Ghorbani (Cronbach’s alpha = 0.83). Data were analyzed using SPSS -16 software. Results: Result showed thinking and mental workload were maximum (81.04 ± 20.49) and frustration score was minimum (47.84 ± 35.29). Mean mental workload among the different age group, different experience and educational degree were not significant different (P>0.05). Mean mental work load among the teacher with two and single job were significantly different (P<0.05). Mean mental workload among the teacher with two and single job were significantly different (P<0.05). Mean mental workload among the teacher with different work hours per week was significantly different (P<0.05). ‍Conclusion: Mean mental workload score was higher among teachers, which can negatively impact the quality and quantity of education by teachers. Thus, implementation of effective programs to mitigate work hours and reduce the mental load in order to enhance teacher’s performance is suggested.